1. Field of the Invention
This invention relates to a slip control system for a vehicle, and more particularly to a slip control system for a vehicle which controls the driving force of the driving wheels when the driving wheels slip due to excessive driving force and controls the braking force applied to the brakes when the wheels tend to lock.
2. Description of the Prior Art
When the driving force of the driving wheels is excessive during acceleration of the vehicle, the driving wheels can slip and the accelerating performance can deteriorate. Thus there has been known a slip control system (traction control system) which, when the slip of the driving wheels relative to the road surface reaches a predetermined value, controls the driving force of the driving wheels so that the slip of the driving wheels converges on a target value. In such a slip control system, the driving force of the driving wheels is controlled by controlling the engine output power and/or applying the braking to the driving wheels.
When the braking force applied to each wheel is excessive during braking, the wheel tends to lock, which can cause skid of the vehicle. Thus there has been known a slip control system (anti-skid brake system) which, when the slip of each wheel relative to the road surface reaches a predetermined value, controls the braking force applied to the wheel so that the slip of the wheel converges on a target value.
Further, as disclosed, for instance, in Japanese Unexamined Patent Publication No. 1(1989)-197160, there has been known a slip control system which controls both the driving force of the driving wheels and the braking force applied to each wheel. For the purpose of simplicity, control of the driving force of the driving wheels will be referred to as "traction control" and control of the braking force for preventing lock of the wheels during braking will be referred to as "anti-skid control", and a slip control system which performs both the traction control and the anti-skid control will be referred to as "total slip control system", hereinbelow.
In such a total slip control system, it is important to avoid interference between the traction control and the anti-skid control in order to prevent malfunction of the system. For this purpose, in the total slip control system disclosed in the above identified patent publication, the traction control is inhibited during the anti-skid control.
However if the traction control is inhibited whenever the anti-skid control is being effected, the following problem arises.
That is, as a situation where the traction control initiating condition is satisfied during the anti-skid control, there can be supposed a case where a driving wheel overrides a projection on the road and comes to race during braking. In such a case, it is preferred that it is preferred that the traction control be immediately initiated. When the traction control which is initiated while the anti-skid control is being effected is by control of the engine output power, there does not arise a problem of interference with the anti-skid control. Further even if the traction control which is initiated while the anti-skid control is being effected is by application of the brake, there does not arise a problem of interference with the anti-skid control so long as the traction control is initiated in the ending of the anti-skid control, i.e., the period (about 2 seconds) from the time at which reduction, hold and increase of the braking pressure are completed to the time at which the final processing in the anti-skid control is completed.
However, even if the traction control is initiated in the manner which does not give rise to a problem of interference with the anti-skid control, interruption handling must be effected when the traction control and the anti-skid control are simultaneously effected. Since the signal processing speed must be increased as the vehicle speed increases, interruption handling causes difficulty. Further since data required for the anti-skid control such as the friction coefficient of the road surface are stored in the ending period of the anti-skid control though control of the braking pressure is not effected, it is preferred that the traction control be not initiated even in the ending period in view of the fact that if the traction control is not effected in the ending period, the anti-skid control can be quickly resumed without calculating the friction coefficient of the road surface and the like.
Thus it is preferred that whether the traction control is to be initiated during the anti-skid control be determined according to the running condition of the vehicle taking into account both improvement in acceleration obtained by initiation of the traction control and improvement in safety by inhibition of initiation of the traction control.